Most cars on the road today have internal combustion engines that require fuel, air, and electricity to run. On modern vehicles, the ignition coil plays an important role in supplying the electrical component to that equation.
It takes the car battery’s low voltage and transforms it into a high voltage to create a spark in the spark plugs, which ignites the air-fuel mixture in the cylinders to power the car.
As such, if you have a faulty ignition coil, your engine will not run properly. Even worse, it may completely stall. As such, it is important to know how to identify bad ignition coil symptoms.
Ignition coils must be in good condition and performing properly for a vehicle’s engine to run correctly. If your engine is not performing like it usually does, a faulty ignition coil may be why.
What are Ignition Coils?
An engine needs four things to run — air, fuel, compression, and spark. Ignition coils take care of the “spark” part, delivering as much as 60,000 volts of power to your vehicle’s spark plugs. How they do this is nothing short of magic.
By opening and closing a circuit in the primary winding of copper wire in the coil, a magnetic field across those windings collapses. Every time the magnetic field collapses, a momentary voltage spike occurs.
This effectively increases 12-volt car battery power to a few hundred volts in the primary windings and tens of thousands in the secondary windings that the spark plugs need to operate.
On older vehicles, a distributor was used to take power from a single coil and distribute it to each spark plug at the correct intervals. While effective for the time, this single coil setup wasn’t as accurate or powerful as the coil-on-plug (COP) design cars use today.
In COP setups, a distributor isn’t necessary to split a single coil’s energy. Instead, each spark plug typically has its own designated ignition coil, and a computer uses various sensors to determine how much power the spark plugs need and when they should fire.
What Do Ignition Coils Do?
To generate power, your vehicle’s engine needs the right mixture of gasoline and oxygen. Then, it needs a spark to ignite the mixture.
Car batteries only have about 12 volts. This is far shy of the several thousand volts necessary to generate the right type of spark. The ignition coil transforms the battery’s voltage to a level that allows the spark plug to do its job.
Older vehicles have a distributor, ignition coil, and spark plug wires. Most vehicles made after the late 1990s typically have a small ignition coil that sits directly on top of each spark plug, which is known as coil-on-plug (COP) design.
Regardless, the functionality is the same. When you start your engine, current flows to the ignition coil. The coil, being a compact electrical transformer, then steps up the voltage level from 12 volts to as high as 60,000.
How Long Do Ignition Coils Last?
Ignition coils generally last 80,000 to 100,000 miles. However, as with most vehicle components, certain conditions like the ones below may cause them to fail sooner.
Incorrect Air-Fuel Ratio
If your fuel-air ratio is out of balance and causing your engine to run too lean or too rich, your spark plugs can develop problems that impact your ignition coils.
This is because mixtures with too much air (lean) may wear out spark plugs sooner since they require more voltage to detonate, generate excessive heat, and are more likely to misfire. On the other hand, mixtures that are too heavy on fuel (rich) can cover spark plugs in carbon deposits.
In both situations, ignition coils are forced to work overtime to deliver enough voltage to combust poor air-fuel mixtures or overcome the damage and grime caused by them.
Incorrect Spark Plug Gap
For spark plugs to operate properly, the space between their electrodes must be gapped correctly.
Too small of a gap, and the spark generated may be too weak to combust fuel well. Too large of a gap, and the spark may not jump the gap consistently or may force ignition coils to deliver more voltage.
Over time, spark plug gaps naturally widen, so it’s important to monitor the health of your spark plugs by paying attention to these six signs your plugs might need to be replaced.
Valve Cover Leaks
Though COP ignition coils may deliver stronger sparks with more accurate timing than single coil setups, their positioning makes them incredibly susceptible to moisture — and it’s no secret that electronics and moisture don’t mix well.
In many COP setups, the coil sits directly above the spark plug, which resides inside a tube in the engine’s valve cover. Over time, the rubber between the spark plug tube and the valve cover may break down and let oil inside of that tube, fouling spark plugs and ignition coils in the process.
Symptoms of a Bad Ignition Coil
When an ignition coil starts to fail, your car is going to give you some clues. These symptoms can range from subtle performance issues to more obvious problems that affect drivability. Recognizing these signs early can help prevent more serious engine damage and costly repairs.
#1. Engine Misfiring.
Engine misfiring is often the first and most noticeable sign of a failing ignition coil. When you start your car, you might hear a sputtering or coughing noise, which tells you that one or more cylinders aren’t firing correctly.
While driving around town, you may notice jerking, hesitation, or a loss of power, especially during acceleration. At idle, the engine might vibrate more than usual or run roughly.
These issues occur because the faulty coil isn’t providing enough voltage to the spark plug, resulting in incomplete combustion in one or more cylinders.
#2. Check Engine Light On/DTC Code.
One of the most reliable indicators of many modern car issues (including ignition coil problems) is the illumination of the check engine light (CEL) on your dashboard. This warning is often accompanied by an error code stored in the vehicle’s computer system.
Scanning for the error code is probably the easiest way to troubleshoot a coil issue, so if you see that CEL, grab your scan tool or have a repair shop confirm.
Fault code P0351 (Ignition Coil A) and P0352 (Ignition Coil B) are two of the most common codes regarding ignition coil problems but other codes may also appear depending on the specific issue and vehicle model.
#3. Poor Fuel Economy.
A decrease in fuel efficiency is another common symptom of a failing ignition coil. When an ignition coil doesn’t work as it should, it can lead to incomplete combustion of fuel in the engine cylinders. As a result, your vehicle may consume more fuel than usual to maintain its performance.
You might notice that you’re filling up your gas tank more frequently or that your miles per gallon have gone down (if that’s something you pay attention to). This increased fuel consumption occurs because the engine is working harder to compensate for the misfire caused by the faulty ignition coil.
#4. Engine Stalling.
Ignition coil failure may also result in the stalling of the vehicle. This can occur because of the irregular sparks sent to the spark plugs by the faulty coil. Your car may shut off completely when brought to a stop leaving you with the trouble of it hopefully restarting.
Engine stalling can be a more severe symptom of ignition coil failure. This happens when the faulty coil isn’t able to provide the necessary spark to keep the engine running.
You might experience your vehicle suddenly shutting off while idling at a stop light or during low-speed driving. In some cases, the engine can be restarted after stalling, but the problem will likely repeat, and you won’t be able to control when that happens.
Having your car stall is particularly dangerous as it can leave you stranded or vulnerable to accidents in traffic. Engine stalling due to ignition coil issues often happens intermittently at first, becoming more frequent as the problem worsens.
#5. Difficulty Starting the Engine.
With a failing ignition coil, starting your vehicle may be more challenging, especially in cold weather. When you turn the key or press the start button, you might notice that the engine takes longer to start or requires multiple attempts before it finally turns over.
This happens because the weak spark from the faulty coil struggles to ignite the fuel-air mixture in the combustion chamber. In colder temperatures, this problem becomes more pronounced as the engine oil thickens and requires more energy to start.
If you find yourself having to crank the engine longer than usual or if it doesn’t start on the first try, particularly on cold mornings, it could be a sign of a deteriorating ignition coil.
#6. Backfiring.
If your vehicle backfires at times, you may have an ignition coil problem on your hands. Backfiring occurs when unburned fuel enters the exhaust system and ignites there instead of in the combustion chamber.
You might hear a loud “pop” or “bang” coming from the exhaust pipe, sometimes accompanied by a puff of black smoke and/or a fuel smell. In some cases, you may literally see flames shooting from the exhaust. This symptom not only affects your vehicle’s performance and makes it annoying loud, but can also damage the exhaust system if you ignore it.
#7. Rough idling.
A faulty ignition coil often makes itself known as rough idling when your vehicle is stationary. When you’re stopped at a traffic light or parked with the engine running, you might notice unusual vibrations or a shaky feeling throughout the car.
The engine may sound uneven, with a choppy or unsteady rhythm instead of a smooth purr. You might also observe fluctuations in the RPM gauge, with the needle wavering more than usual.
This rough idling occurs because the failing ignition coil isn’t providing consistent spark to one or more cylinders, causing the engine to run unevenly. If your car starts to feel more like a massage chair than a smooth-running machine when idling, it’s worth checking the ignition coils.
#8. Poor Power and Performance.
During acceleration, the car might feel sluggish or unresponsive, struggling to gain speed when you press the gas pedal. You may notice hesitation or jerking, especially when trying to climb a hill or merge onto a highway.
At higher speeds, the engine might not maintain power consistently, leading to difficulties in passing other vehicles. Additionally, you could experience intermittent power loss, where the car briefly loses power and then regains it, creating a sporadic driving experience.
While experiencing these power-related problems doesn’t necessarily mean you have a bad ignition coil (there are many other causes), when you notice this symptom along with others above, the odds of a coil being at fault increase.
Ignition Coil Replacement Cost
The cost of a new ignition coil depends on the make and model of the car. Some coils are as cheap as $75 while others cost in the $300 range. If you have the replacement professionally done then the labor costs will be between $50 and $100 per hour.
Therefore, you can expect to pay at least $150 to $200 if you were to take your car to an automotive repair shop and have them replace your ignition coil for you. If you choose to go to a dealership, expect to pay even more.
How to Test an Ignition Coil
Here are some tips on testing an ignition coil based on whether they are a CNP (Coil-Near-Plug) or COP (Coil-On-Plug).
CNP Coil Type
- To run a test on an ignition coil, first turn off your car’s engine and open up the hood. Remove or pull the spark plug wire from it (if you car uses CNP coils). These wires typically start from the distributor cap and run to the spark plug. Use rubber gloves and insulated tools when working with these electrical components or you may get a nice jolt.
- Now attach the new spark plug to the spark plug wire (new or old spark plug to test coil sparks). Use insulated pliers to hold the spark plug onto some metal part of the engine so that the threaded portion of the spark plug is touching the metal.
- Use a fuse removal tool or needle nose pliers to remove the fuse to the fuel pump to disable it and get ready to crank the engine. You may need an extra person to turn the key in the ignition because you are holding the spark plug down with the pliers.
- Once the engine is cranked, look for blue sparks forming along the spark plug gap. If you see blue sparks then your ignition coil is working properly.
- If you don’t see the sparks or if you see orange sparks then this is a sign of the ignition coil malfunctioning.
- When you’re done with the test, disconnect the spark plug, place it back in its hole, reconnect the spark plug wires to it, and put back the fuel pump fuse.
COP Coil Type
- Start the engine and keep it running at idle speed.
- Locate the first ignition coil (they’re typically arranged in order along the engine).
- With the engine still running, carefully disconnect the electrical connector from the first coil.
- Observe the engine’s behavior. If this coil was working properly, you should notice an immediate change in the engine’s running. It should become rougher or even stall.
- If there’s no change when you disconnect the coil, this likely indicates that coil was already bad.
- Reconnect the coil before moving to the next one.
- One coil at a time, repeat the same disconnection process.
- Make note of any coils that, when disconnected, don’t cause a change in the engine’s running. Those are your likely culprits.
How Do You Replace a Damaged Ignition Coil?
Many ignition coils offer a plug-and-play design, making them easy to replace. If your vehicle has this style of coils, even a relatively new DIYer can likely handle the replacement without professional assistance. Still, some coils are hard to access.
If you do the job on your own, though, you only pay the parts costs. While the price of replacement ignition coils varies widely, on most vehicles, it ranges from $40 to $300 for parts. If you have a mechanic do the job, you must pay labor costs.
These likely depend on the design of your engine and the location of the bad ignition coil. The mechanic will be able to clear fault codes after the repair.